1. Field of the Invention
This invention relates to a fastener and relates particularly to a fastener providing the tensile force and attaining an anti-loosening effect.
2. Description of the Related Art
Referring to FIG. 1 to FIG. 3, a U.S. Patent Application Publication No. 2005/0186048 discloses a thread-forming screw fastener 1 which comprises a shank 11, a head 12 disposed on the shank 11, a drilling portion 13 disposed on the shank 11, opposite to the head 12, and a plurality of threads 14 spirally disposed on the shank 11. Further, each of the threads 14 includes a plurality of connective wave troughs 141 disposed thereon. Each of the threads 14 has an upper surface 142 and a lower surface 143 opposite to the upper surface 142. The two surfaces 142, 143 converge at a wave crest 145. The wave crest 145 defines an included angle 144. Each wave trough 141 defines a trough angle 146. Accordingly, when the fastener 1 starts to drill, the wave troughs 141 execute a cutting action to cut debris of an object (not shown) to complete the fastening operation.
However, the disclosed fastener 1 still has problems. First, the threads 14 may easily be broken. Generally, the cutting efficiency of the threads 14 depends on the number of the trough angle 146. For example, if the trough angle 146 is too small, the cut debris provided with the elastic property may not enter the wave troughs 141 efficiently when the threads 14 carry out a speedy drilling action. This renders the wave troughs 141 unable to provide an auxiliary cutting for the threads 14. On the other hand, if the trough angle 146 is too large, the size of the threads 14 may be reduced to become a thinner structure. Such thinner structure is unable to effectively bear the drilling torque and is easily broken. Second, the drilling torsion cannot be reduced. Generally, the threads 14 are formed on the shank 11 by using a thread-rolling die (not shown). When the threads 14 are rolled and formed, the surface of the shank 11 becomes ragged or uneven. Thus, the shank 11 with the ragged surface may twist too much and may even snap when it cannot bear the drilling stress. Moreover, the drilling efficiency of the fastener 1 is affected by the number of the included angle 144 of each thread 14. For example, although the smaller included angle 144 provides a better cutting efficiency, the threads 14 are still unable to bear a larger screw-in torque at the time of drilling into a rigid object. Thus, the threads 14 get broken easily, and an uncompleted drilling action is incurred. On the other hand, although the larger included angle 144 provides a better bearing force for the threads 14, the cutting efficiency of the larger included angle 144 is still worse than the cutting efficiency of the smaller included angle 144 and the uncompleted drilling still occurs. Thus, the fastener 1 still needs to be improved.